FILM PRODUCTION METHOD, BATTERY SEPARATOR FILM, NONAQUEOUS ELECTROLYTE SECONDARY BATTERY SEPARATOR, AND NONAQUEOUS ELECTROLYTE SECONDARY BATTERY
A desired drying capability is achieved while damage to a film is prevented. A film production method is arranged such that: a production process including a drying step is operated by setting a drying condition, under which to carry out the drying step, for each of at least two periods, the two periods being a first period and a second period later than the first period; the drying condition is changed in at least a part of the first period so as to be enhanced with time; and the drying condition is maintained in the second period so as to be substantially fixed.
1 . A film production method comprising a drying step of drying a film, wherein:
a production process including the drying step is operated by setting a drying condition, under which to carry out the drying step, for each of at least two periods, the two periods being a first period and a second period later than the first period;
the drying condition is changed in at least a part of the first period so as to be enhanced with time; and
the drying condition is maintained in the second period so as to be substantially fixed.
2 . The film production method as set forth in claim 1 , wherein a decline in drying capability demonstrated in the drying step, the decline being caused by absorption of heat during the drying of the film, is prevented by changing the drying condition in the first period so that the drying condition is enhanced.
3 . The film production method as set forth in claim 2 , wherein:
the drying capability is maintained constant in the first period by changing the drying condition in the first period so that the drying condition is enhanced;
the drying capability is maintained constant in the second period by causing the absorption of heat and application of heat to the drying step to reach equilibrium at an end of the first period; and
the drying capability is consequently constant throughout the first period and the second period.
4 . The film production method as set forth in claim 1 , wherein a time at which the first period starts is set so that a lead part of the film which is to be formed into a product starts to be subjected to the drying step at the time.
5 . The film production method as set forth in claim 1 , wherein:
the drying is carried out by bringing the film into contact with a roller that is heated; and
the drying condition is a temperature of a heating medium to be poured into the roller.
6 . The film production method as set forth in claim 1 , wherein the operation of the production process is a restart of the production process which is stopped.
7 . The film production method as set forth in claim 1 , wherein the film is a battery separator.
8 . The film production method as set forth in claim 1 , wherein in the drying step, deformation in the film is prevented by carrying out a heat treatment so as to cause shrinkage not only in a functional layer of the film but also in a base material of the film.
9 . A battery separator film, wherein:
in a case where a curling amount W is found at 12 positions which are arranged in a longer side direction of the battery separator film at intervals of an integral multiple of 100 m and in which a distance between positions that are most distant from each other is not shorter than 100 m and not longer than 300 m,
a coefficient of variation σ with respect to the found curling amount W is not more than 0.15,
the curling amount W being represented by the following equation (1):
curling amount W=W 1− W 2 (1)
where W1 is a width of a film that has been cut out in the longer side direction, and W2 is a projection width of a part of the film which part is smallest in projection width when seen from a direction perpendicular to a surface of the film while the film, to which a tension of 90 N/m is applied, is stretched between two rollers that are provided in parallel with each other at intervals of 27.5 cm, and
the coefficient of variation a being represented by the following equation (2):
coefficient of variation σ−standard deviation of curling amount W /average of curling amount W (2)
10 . The battery separator film as set forth in claim 9 , wherein the curling amount W which is found for a length of not shorter than 100 m and not longer than 2000 m has a maximum value of not more than 0.3 at any of positions that are arranged in the longer side direction at intervals of 100 m.
11 . A nonaqueous electrolyte secondary battery separator comprising a battery separator film recited in claim 9 .
12 . A nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte secondary battery separator recited in claim 11 .